High repetition rate laser produced plasma EUV light source

ABSTRACT

An EUV light source apparatus and method are disclosed, which may comprise a pulsed laser providing laser pulses at a selected pulse repetition rate focused at a desired target ignition site; a target formation system providing discrete targets at a selected interval coordinated with the laser pulse repetition rate; a target steering system intermediate the target formation system and the desired target ignition site; and a target tracking system providing information about the movement of target between the target formation system and the target steering system, enabling the target steering system to direct the target to the desired target ignition site. The target tracking system may provide information enabling the creation of a laser firing control signal, and may comprise a droplet detector comprising a collimated light source directed to intersect a point on a projected delivery path of the target, having a respective oppositely disposed light detector detecting the passage of the target through the respective point, or a detector comprising a linear array of a plurality of photo-sensitive elements aligned to a coordinate axis, the light from the light source intersecting a projected delivery path of the target, at least one of the which may comprise a plane-intercept detection device. The droplet detectors may comprise a plurality of droplet detectors each operating at a different light frequency, or a camera having a field of view and a two dimensional array of pixels imaging the field of view. The apparatus and method may comprise an electrostatic plasma containment apparatus providing an electric plasma confinement field at or near a target ignition site at the time of ignition, with the target tracking system providing a signal enabling control of the electrostatic plasma containment apparatus. The apparatus and method may comprise a vessel having and intermediate wall with a low pressure trap allowing passage of EUV light and maintaining a differential pressure across the low pressure trap. The apparatus and method may comprise a magnetic plasma confinement mechanism creating a magnetic field in the vicinity of the target ignition site to confine the plasma to the target ignition site, which may be pulsed and may be controlled using outputs from the target tracking system.

RELATED APPLICATIONS

The present application is related to a co-pending application AttorneyDocket Ser. No. ______ 2003-0083-01, COLLECTOR FOR EUV LIGHT SOURCE,filed on Mar. 10, 2004, the disclosure of which is hereby incorporatedby reference.

FIELD OF THE INVENTION

The present invention relates to a system for the generation of EUVlight using a laser produced plasma and discrete targets in the form ofsolid particles or droplets or solid particles imbedded in a dropletdelivered to an irradiating site for irradiation by a pulsed laser beam.

BACKGROUND OF THE INVENTION

LPP EUV sources have been under discussion for some time. As therequirements for, e.g., smaller and smaller integrated circuit criticaldimension lithography and the concomitant requirement for shorter andshorter wavelength light sources, in the ranges of tens of tens ofnanometers (e.g., 10-30), the need for a workable EUV light source thatcan also meet all of the requirements for power, repetition rate, dosestability, and the like requirements the actual requirements for an EUVlight source, e.g., for use as a lithography light source, are becomingmore clear. By way of example, there are some indications of what thepower requirements could be. One way to look at this is to comparereported performance of a laser produced plasma (“LPP”) system, e.g. theTRW/CEO, system, incorporating certain lithography parameters thatappear to be system requirements, with proposals for a deep plasma focussystem, a variety of discharge produced plasma “(DPP”) systems. Reportednumbers for the TRW/CEO system are shown below in Table I. TABLE ITRW/CEO LPP Collected EUV power at 100 W** intermediate focus (“I.F.”)Collector optical transmission 55%* EUV power into collector 181 WGeometric collection efficiency 5 str/2π str EUV power into 2π str 227 WLaser-to-EUV conversion 1.0% “Pump” power into vessel 22,700 WElectrical-to-laser conversion 3% Wall plug electrical power 756,666 W*According to a TRW/CEO poster paper given at the 2003 SPIE.**According to requirements being stated by potential customers for EUVlight sources.

While some systems in use, e.g., in an integrated circuit fabricationfacility require power in the range of a kilowatt, the likelihood isthat there would be required many more scanners using EUV light sourcesper fab than, e.g., ion implanters or rapid thermal annealing systems,also requiring this type of projected input power. There is a clear needfor improvements to proposals for EUV light source efficiencies.

One area of critical importance to the overall efficiency of such an EUVlight source is the collector. Many issues of collector efficiency needto be addressed, including debris management, which can interfere withthe ability to deliver the required light energy to the intermediatefocus and also decrease economic efficiency of the light source ifdebris, e.g., requires frequent replacement of the collector due toinability to control debris deposition over time. Proposals for acollector system have been discussed in the co-pending applicationentitled COLLECTOR FOR EUV LIGHT SOURCE, filed on Mar. 10, 2004,Attorney Docket No. 2003-0083-01, the disclosure of which is herebyincorporated by reference.

With, e.g., a 10% electrical-to-laser conversion efficiency then therequired wall plug power becomes 227,000 W. This value is essentiallythe same as for the discharge produced plasma (“DPP”). If TRW/CEO canalso achieve their stated goal of doubling the laser-to-EUV efficiency,then the required wall plug power becomes 113,500 W. Of course, themethods of increasing this conversion efficiency will likely apply tothe DPP and thus the DPP wall plug requirements will also drop by half.

One of the driving forces behind the design of an EUV lithography lightsource and, e.g., the selection of target material, collector strategy,discharge produced plasma (“DPP”, e.g., deep plasma focus (“DPF”) orlaser produced plasma (“LPP”) and the like is the requirement by thelithography tool manufacturers regarding the level of out-of-bandradiation, e.g., produced by an LPP source, e.g., with a 248 nm drivelaser. Since the EUV multi-layer mirrors exhibit high reflectivity tothe UV region and many of the proposed EUV photoresists are sensitive toUV/DUV, it is critical that the source does not produce a large amountof radiation, e.g., in the 130-400 nm range. With a 248 nm drive laser,as opposed to an infrared drive laser, even a small amount of scatteredlaser light may lead to high levels of UV radiation from the EUV source.

The currently contemplated full specification for out-of-band radiationfor a production EUV source is listed below in the wavelength ranges ofinterest and the allowed ratio to the in-band, e.g., at 13.5 nm energy.Allowed Percentage Range (relative to 13.5 nm in-band)  10-40 nm 100% 40-130 nm 100% 130-400 nm  1% 400-800 nm 100%   >800 nm 0.05% Therefore all radiation, e.g., between 130 nm and 400 nm must be lessthan 1% of the in-band 13.5 nm radiation. Thus, if one assumes, e.g., a2% contribution into in-band EUV then one must also have only a 0.02%conversion efficiency into the 130-400 nm band. This is an incrediblytight requirement, for both LPPs and DPPs.

Behavior of expanding laser produced plasma and/or the effects ofmagnetic fields on plasmas have been modeled and studied, as discussed,e.g., in H. Pant, “Behavior of Expanding Laser Produced Plasma in aMagnetic Field,” Physica Scripta, Vol. T75 (1998), pp. 104-111;Tillmack, Magnetic Confinement of LPP, UCSD Report and Abramova,“Tornado Trap”, the disclosures of which are hereby incorporated byreference.

SUMMARY OF THE INVENTION

An EUV light source apparatus and method are disclosed, which maycomprise a pulsed laser providing laser pulses at a selected pulserepetition rate focused at a desired target ignition site; a targetformation system providing discrete targets at a selected intervalcoordinated with the laser pulse repetition rate; a target steeringsystem intermediate the target formation system and the desired targetignition site; and a target tracking system providing information aboutthe movement of target between the target formation system and thetarget steering system, enabling the target steering system to directthe target to the desired target ignition site. The target trackingsystem may provide information enabling the creation of a laser firingcontrol signal, and may comprise a droplet detector comprising acollimated light source directed to intersect a point on a projecteddelivery path of the target, having a respective oppositely disposedlight detector detecting the passage of the target through therespective point, or a detector comprising a linear array of a pluralityof photo-sensitive elements aligned to a coordinate axis, the light fromthe light source intersecting a projected delivery path of the target,at least one of the which may comprise a plane-intercept detectiondevice. The droplet detectors may comprise a plurality of dropletdetectors each operating at a different light frequency, or a camerahaving a field of view and a two dimensional array of pixels imaging thefield of view. The apparatus and method may comprise an electrostaticplasma containment apparatus providing an electric plasma confinementfield at or near a target ignition site at the time of ignition, withthe target tracking system providing a signal enabling control of theelectrostatic plasma containment apparatus. The apparatus and method maycomprise a vessel having and intermediate wall with a low pressure trapallowing passage of EUV light and maintaining a differential pressureacross the low pressure trap. The apparatus and method may comprise amagnetic plasma confinement mechanism creating a magnetic field in thevicinity of the target ignition site to confine the plasma to the targetignition site, which may be pulsed and may be controlled using outputsfrom the target tracking system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of an overall broad conception for alaser-produced plasma EUV light source according to an aspect of thepresent invention;

FIG. 1A shows schematically the operation of the system controlleraccording to an aspect of an embodiment of the present invention;

FIG. 2A shows a side view of an embodiment of an EUV light collectoraccording to an aspect of the present invention looking from anirradiation ignition point toward an embodiment of a collector accordingto an embodiment of the present invention;

FIG. 2B shows a cross-sectional view of the embodiment of FIG. 2A alongthe lines 2B in FIG. 2A;

FIG. 3 shows in schematic form a possible embodiment of a targetdelivery system according to an aspect of an embodiment of the presentinvention;

FIGS. 4A and B show schematically a possible embodiment of a targettracing system according to an aspect of an embodiment of the presentinvention, with FIG. 4A being a schematic side view of an aspect of theembodiment and FIG. 4B being a plan view of an aspect of the embodiment;

FIG. 5 shows a schematic perspective view of aspects of an alternativeembodiment of a target tracking system according to an aspect of anembodiment of the present invention;

FIG. 6 shows a cross-sectional view according to an aspect of anembodiment of the present invention including cold fingers for debriscollection;

FIGS. 7A-C there is shown an apparatus and method for electrostaticallyconfining a, plasma, e.g., a laser produced plasma according to anaspect of an embodiment of the present invention;

FIGS. 8A-G there is shown schematically aspects of an embodiment of thepresent invention;

FIG. 9 there is shown a block diagram of an aspect of an embodiment ofthe present invention regarding feedback and control; and,

FIG. 10 shows aspects of an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to FIG. 1 there is shown a schematic view of an overallbroad conception for an EUV light source, e.g., a laser produced plasmaEUV light source 20 according to an aspect of the present invention. Thelight source 20 may contain a pulsed laser system 22, e.g., a gasdischarge excimer or molecular fluorine laser operating at high powerand high pulse repetition rate and may be a MOPA configured lasersystem, e.g., as shown in U.S. Pat. Nos. 6,625,191, 6,549,551, and6,567,450. The light source 20 may also include a target delivery system24, e.g., delivering targets in the form of liquid droplets, solidparticles or solid particles contained within liquid droplets. Thetargets may be delivered by the target delivery system 24, e.g., intothe interior of a chamber 26 to an irradiation site 28, otherwise knownas an ignition site or the sight of the fire ball. Embodiments of thetarget delivery system 24 are described in more detail below.

Laser pulses delivered from the pulsed laser system 22 along a laseroptical axis 55 through a window (not shown) in the chamber 26 to theirradiation site, suitably focused, as discussed in more detail below incoordination with the arrival of a target produced by the targetdelivery system 24 to create an ignition or fire ball that forms anx-ray releasing plasma, having certain characteristics, includingwavelength of the x-ray light produced, type and amount of debrisreleased from the plasma during or after ignition, according to thematerial of the target.

The light source may also include a collector 30. e.g., a reflector,e.g., in the form of a truncated ellipse, with an aperture for the laserlight to enter to the ignition site 28. Embodiments of the collectorsystem are described in more detail below. The collector 30 may be,e.g., an elliptical mirror that has a first focus at the ignition site28 and a second focus at the so-called intermediate point 40 (alsocalled the intermediate focus 40) where the EUV light is output from thelight source and input to, e.g., an integrated circuit lithography tool(not shown). The system 20 may also include a target position detectionsystem 42. The pulsed system 22 may include, e.g., a masteroscillator-power amplifier (“MOPA”) configured dual chambered gasdischarge laser system having, e.g., an oscillator laser system 44 andan amplifier laser system 48, with, e.g., a magnetic reactor-switchedpulse compression and timing circuit 50 for the oscillator laser system44 and a magnetic reactor-switched pulse compression and timing circuit52 for the amplifier laser system 48, along with a pulse power timingmonitoring system 54 for the oscillator laser system 44 and a pulsepower timing monitoring system 56 for the amplifier laser system 48. Thesystem 20 may also include an EUV light source controller system 60,which may also include, e.g., a target position detection feedbacksystem 62 and a firing control system 65, along with, e.g., a laser beampositioning system 66.

The target position detection system may include a plurality of dropletimagers 70, 72 and 74 that provide input relative to the position of atarget droplet, e.g., relative to the ignition site and provide theseinputs to the target position detection feedback system, which can,e.g., compute a target position and trajectory, from which a targeterror can be computed, if not on a droplet by droplet basis then onaverage, which is then provide as an input to the system controller 60,which can, e.g., provide a laser position and direction correctionsignal, e.g., to the laser beam positioning system 66 that the laserbeam positioning system can use, e.g., to control the position anddirection of the laser position and direction changer 68, e.g., tochange the focus point of the laser beam to a different ignition point28.

The imager 72 may, e.g., be aimed along an imaging line 75, e.g.,aligned with a desired trajectory path of a target droplet 94 from thetarget delivery mechanism 92 to the desired ignition site 28 and theimagers 74 and 76 may, e.g., be aimed along intersecting imaging lines76 and 78 that intersect, e.g., alone the desired trajectory path atsome point 80 along the path before the desired ignition site 28.

The target delivery control system 90, in response to a signal from thesystem controller 60 may, e.g., modify the release point of the targetdroplets 94 as released by the target delivery mechanism 92 to correctfor errors in the target droplets arriving at the desired ignition site28.

An EUV light source detector 100 at or near the intermediate focus 40may also provide feedback to the system controller 60 that can be, e.g.,indicative of the errors in such things as the timing and focus of thelaser pulses to properly intercept the target droplets in the rightplace and time for effective and efficient LPP EUV light production.

Turning now to FIG. 1A there is shown schematically further details of acontroller system 60 and the associated monitoring and control systems,62, 64 and 66 as shown in FIG. 1. The controller may receive, e.g., aplurality of position signal 134, 136 a trajectory signal 136 from thetarget position detection feedback system, e.g., correlated to a systemclock signal provided by a system clock 116 to the system componentsover a clock bus 115. The controller 60 may have a pre-arrival trackingand timing system 110 which can, e.g., compute the actual position ofthe target at some point in system time and a target trajectorycomputation system 112, which can, e.g., compute the actual trajectoryof a target drop at some system time, and an irradiation site temporaland spatial error computation system 114, that can, e.g., compute atemporal and a spatial error signal compared to some desired point inspace and time for ignition to occur.

The controller 60 may then, e.g., provide the temporal error signal 140to the firing control system 64 and the spatial error signal 138 to thelaser beam positioning system 66. The firing control system may computeand provide to a resonance charger portion 118 of the oscillator laser44 magnetic reactor-switched pulse compression and timing circuit 50 aresonant charger initiation signal 122 and may provide, e.g., to aresonance charger portion 120 of the PA magnetic reactor-switched pulsecompression and timing circuit 52 a resonant charger initiation signal,which may both be the same signal, and may provide to a compressioncircuit portion 126 of the oscillator laser 44 magnetic reactor-switchedpulse compression and timing circuit 50 a trigger signal 130 and to acompression circuit portion 128 of the amplifier laser system 48magnetic reactor-switched pulse compression and timing circuit 52 atrigger signal 132, which may not be the same signal and may be computedin part from the temporal error signal 140 and from inputs from thelight out detection apparatus 54 and 56, respectively for the oscillatorlaser system and the amplifier laser system.

The spatial error signal may be provided to the laser beam position anddirection control system 66, which may provide, e.g., a firing pointsignal and a line of sight signal to the laser bean positioner whichmay, e.g. position the laser to change the focus point for the ignitionsite 28 by changing either or both of the position of the output of thelaser system amplifier laser 48 at time of fire and the aiming directionof the laser output beam.

Turning now to FIGS. 2A and 2B there is shown, respectively a schematicview side view of a collector 30 looking into the collector mirror 150,and a cross-sectional view of the rotationally symmetric collectormirror 150 arrangement along cross-sectional lines 2B in FIG. 2A(although the cross-sectional view would be the same along any radialaxis in FIG. 2A.

As shown in FIG. 2A the elliptical collection mirror 150 is circular incross section looking at the mirror, which may be the cross-section atthe greatest extension of the mirror, which is shown in FIG. 1A to bealmost to the focus point 28 of the elliptical mirror 150, so as not toblock target droplets 94 from reaching the ignition point designed to beat the focus point 28. It will be understood, however, that the mirrormay extend further towards the intermediate focus, with a suitable holein the mirror (not shown) to allow passage of the target droplets to thefocus point. The elliptical mirror may also have an aperture 152, e.g.,shown to be circular in FIG. 2A, to allow entry of the LPP laser beam154, e.g., focused through focusing optics 156, through the mirror 150to the ignition point 28 desired to be at the focus of the ellipticalmirror. The aperture 152 can also be, e.g., more tailored to the beamprofile, e.g., generally rectangular, within the requirements, if any ofmodifying the beam optical path to make corrections of the focus of thelaser beam 154 on an ignition site, depending upon the type of controlsystem employed.

Also shown in FIGS. 2A and 2B is a debris shield 180 according to anaspect of an embodiment of the present invention. The debris shield 180may be made up of a plurality of thin plates 182, made, e.g., of thinfoils of, e.g., molybdenum, extending radially outward from the desiredignition site and defining narrow planar radially extending channels 184through the debris shield 180. The illustration of FIG. 2A is veryschematic and not to scale and in reality the channels are as thin ascan possibly be made. Preferably the foil plates 182 can be made to beeven thinner than the channels 184, to block as little of the x-raylight emitted from the plasma formed by ignition of a target droplet 94by the laser beam 155 focused on the ignition site 28.

Seen in cross section in FIG. 2B, the functioning of the channels 182 inthe debris shield 180 can be seen. A single radial channel is seen inFIG. 2B and the same would be seen in any section of the collector 30through the rotationally symmetric axis of rotation of the collectormirror 150 and debris shield 180 within a channel of the debris shield180. Each ray 190 of EUV light (and other light energy) emitted from theignition site 28 traveling radially outward from the ignition site 28will pass through a respective channel 182 in the debris shield 180,which as shown in FIG. 2B may, if desired, extend all the way to thecollection mirror 150 reflective surface. Upon striking the surface ofthe elliptical mirror 150, at any angle of incidence, the ray 190 willbe reflected back within the same channel 180 as a reflected ray 192focused on the intermediate focus 40 shown in FIG. 1.

Turning now to FIG. 3 there is shown a possible embodiment of a targetformation/delivery system 24 according to an aspect of an embodiment ofthe present invention. The target deliver system 24 may comprise, e.g.,a target formation/delivery apparatus 200, which may have, e.g., a body202 and a cap 204, with the body 202 and the cap 204, e.g., defining aninterior cavity 206 which may contain target material, e.g., lithium,e.g., in a relatively pure state and, e.g., in a liquid form or even asolid form, e.g., relatively uniform radius pellets of, e.g., about 20μm in diameter. As illustrated in FIG. 3, the source is lithium inliquid form which may be fed to the cavity 206, e.g., in liquid or solidform through a source input (not shown) and may, e.g., be kept underpressure of, e.g., 10-20 psi, for, e.g., for liquid tin as a target, andlikely much less for lithium, based on the difference in mass andviscosity between tin and lithium, through a source 212 for, e.g.,pressurizing gas, which may be, e.g., argon.

The target formation/delivery apparatus 200 may also have heaters, e.g.,cartridge heaters 210, e.g., annularly surrounding the body 202 andserving to, e.g., heat the body to, e.g., maintain the liquid targetmaterial, e.g., liquid lithium in liquid form, e.g., by maintaining thematerial in the cavity at or above, e.g., 500° C. for lithium.

The cavity 206 at, e.g., its lower end may open into a nozzle 220, whichmay have a narrowing portion 222, which may serve, e.g., in thealternative embodiment of a solid target pellet source to narrow down toessentially the size of one target pellet before a nozzle opening 226 atthe terminal end of the nozzle 220, and in the case of the embodimentusing liquid target material, narrowing down to a size that essentiallydefines a stream 220 of about, e.g., 20 μm in diameter, which can serve,e.g., to separate into target droplets 94.

Target droplets 94 may be formed, e.g., through the use of a perturber226, which may, e.g., under the influence of a signal from a targetdelivery system controller 90, e.g., a periodic signal, e.g., a signwave as indicated schematically in FIG. 3, squeeze the nozzle to addperturbing discontinuities into the liquid stream 224, which can, e.g.,select the size and distribution of the target drops 94 that actuallyform eventually from the stream 224. The target delivery controller may,in turn be controlled from the overall system controller 60.

The overall system controller 60 may also control a target deliverysystem position controller 240, e.g., based on information supplied tothe overall system controller 60 regarding position error of apreviously delivered target droplet or droplets in regard to, e.g., adesired ignition site. The position controller 240 may translate thetarget formation/delivery apparatus, e.g., in a plane orthogonal to theaxis of the output stream 224 to, e.g., adjust the location of thenozzle output 226 in that plane. This may be done by servo motors orpiezoelectric actuators or a combination of both, e.g., for a slowaiming control loop and a faster speed aiming control loop or, e.g.,course and fine aiming control.

Applicants have noted in experiments that that in delivering, e.g., a 20μm diameter droplet to a desired target simulated ignition site over adistance of, e.g., about 50 μm (larger distances may also be needed forprotection of, e.g., the nozzle from the plasma and its debris, an errorof, e .g., about 0.25 mm can occur in the arrival point vis-á-vis thedesired target ignition site. Applicants believe that this is due to thedroplet initially leaving the nozzle of the target formation apparatus200 at an angle to the correct trajectory path to the target site,normally true vertical (as shown illustratively in FIG. 3). Applicantsalso believe that this may be due to some effects such as lateraldifferences in temperature or the like across the nozzle opening, whichmay be relatively steady state once formed. To this effect, applicantspropose that a tilting mechanism (not shown) e.g., incorporated in thetarget formation system 92 position controller 240, to tilt the nozzleequally and oppositely away from the droplet formation axis tilt error,based, e.g., on feedback of target position error signals measuring theeffect of this droplet formation axis tilt error to remove the error intarget arrival position, e.g., vis-á-vis the target ignition site. Thismay be done, e.g., with piezoelectric elements, which may only need toinduce a tilt of, e.g., 5-10 steradians in the nozzle to counteract thedroplet formation axis error at the nozzle output for a correct flightpath to the target ignition site.

The overall system controller 60 may also provide a signal (not shown)to the target delivery system 92 to control the pressure of the, e.g.,argon pressurizing gas which may, e.g., serve to adjust the size of theultimate droplets 94, the delivery rate of the droplets 94, the spacingof the droplets 94, or some other operating parameter of theformation/delivery of the droplets 94 to the desired ignition site 28 orto a target tracking and steering system 350, discussed in more detailbelow, for ultimate delivery to the ignition site 28.

Turning now to FIGS. 4A and B there are shown aspects of an embodimentof a possible target tracking system 42 according to an aspect of anembodiment of the present invention. The target tracking system 42 maycomprise, e.g., a helium-neon laser (HeNe) laser 250, selected, e.g.,for its relatively inexpensive nature. The HeNe laser may produce a beam256 of light at a wavelength/frequency of, e.g., 632-38 nm, and may bedelivered to an optic 252 that is, e.g., also impinged by the laserlight source 22 beam 154 and may be, e.g., essentially fullytransmissive of the beam 154 and may, e.g., reflect part of the beam256, e.g., through the same focusing optics 156 as for the beam 154,i.e., focused to the desired ignition spot 28.

The target tracking system 42 may also include, e.g., another focusingoptic 260 that may, e.g., focus the light passing through the focuspoint at the ignition site 28 onto, e.g., a detector 262. The detector262 may be, e.g., a photodiode or an array of photodiodes, e.g., alinear array of photodiodes, selected to be sensitive to light in theband of the HeNe laser and not in the band of the laser 22. The detector262 may, e.g., provide an output signal, a high or a low, each time,e.g., the light from the HeNe laser 250, to which it is selectivelysensitive, is cut off, e.g., to one or more photo-diodes of thedetector, e.g., by the passage of a droplet 94 into the path of thelight from the HeNe laser, e.g., at or near the ignition point 28.

It will be understood that the detector may comprise, e.g., a lineararray of photo-diodes sensitive to the wavelength of the HeNe laser andprovide to the controller 60 or to some feedback system, e.g., positionfeedback system 62, a signal or signals that can be analyzed todetermine some displacement in the array, e.g., in the direction towardor away from the lateral array or across the array, e.g., in the lateralaxis of the array, indicating, e.g., the passage of a target dropletthrough or on either side of the true ignition site 28 in, e.g., someplane, e.g., a horizontal plane (so oriented as shown in FIG. 4A,assuming that the horizontal plane is oriented orthogonal to the planeof the paper) through the ignition site 28.

It will also be understood that if the detector 262 includes anotherlinear array of photo-diodes, e.g., oriented vertically (as shown in thefigure) the some distribution of intensity signals from the array may beused, e.g., to determine a lateral displacement of the droplet from theignition site, e.g., as shown illustratively at positions 94 a and 94 bin FIG. 4A.

Barring the ability to discern such an error displacement horizontallyor vertically from varying intensities or displacement of an intensitysignal to, e.g., other than a central photo-diode in such an array(horizontal or vertical), then displacement of the droplet, e.g., asshown in FIG. 4A to either the position 94 a or 94 b may simply give afalse indication that the droplet 94 is on target, e.g., if enough ofthe HeNe light is blocked from the detector, even by an erroneouslylocated droplet, e.g., as shown schematically and not to scale bypositions 94 a and 94 b. Then the output signal of the photodiode(s) inthe detector 262 may be still interpreted to be the low (or high) signalmentioned above indicating the target droplet 94 to be at the ignitionsite 28.

Referring to FIG. 4B, there is shown another possible arrangementaccording to aspects of an embodiment of the present invention that mayserve to alleviate this possible error in the tracking system 42operation, by, e.g., requiring a plurality of such intersected signals,e.g., two or three to indicate the droplet 94 has intersected theignition site 28. The embodiment of FIG. 4B may again incorporate thebeam 256 as shown in FIG. 4A passing through the irradiating laser beam154 focusing optic 156, as explained in regard to FIG. 4A. One advantageof passing through this optic 156, is that the HeNe beam 256 is alwaysfocused to the desired ignition spot 28, assuming there is feedback, asdiscussed in more detail below that uses, e.g., the focusing optic 156,by either moving the optic 156 or, if possible and convenient, movingthe laser 22, or using beam pointing equipment as discussed in moredetail below, all to, e.g., focus to an ignition site 28, e.g.,according to where the droplets 94 are being delivered by the targetdeliver system 92 as discussed above and/or target tracking and steeringsystem 350 as discussed below.

The embodiment of FIG. 4B may also comprise, e.g., at least oneadditional target tracking laser system, e.g., delivering a laser bean,e.g., from a HeNe laser, e.g., 256 a and 256 b to another focusingoptic, e.g., 260 a and 260 b, respectively focused on another detector,e.g., 262 a and 262 b, respectively. In this manner, two or more low (orhigh) signals must be received by the feedback system 62, to indicatethat the droplet 94 has passed through the ignition site from, e.g., twoadditional angles of imaging the ignition site 28. As explained above,the respective detectors 262, 262 a and 262 b may have, e.g., a lineararray or orthogonal linear arrays of photo-detectors that may provideintensity data in the photodiodes of such array(s) that can be used todetermine position errors, horizontally or vertically or both of thedroplet 94 in relation to the desired ignition site. This may evenenable the intensity data to be used to detect position error of thedroplet from an ignition site 28′ (not shown) different from some fixeddesired ignition site, e.g., if the laser 22 is focused to the new site28′ due to target delivery system error in delivering the targetdroplets to the fixed desired ignition site, i.e., perfectly on thefocus of the collector.

It will be also understood that one of the HeNe laser beans 256, 256 aor 256 b may be oriented to be above the plane of the paper asillustrated in FIG. 4B such that it may detect the passage of a targetdroplet through a location (not shown) prior to reaching the ignitionsite 28. This may be used, e.g., by the feedback controller 62 and/orthe main controller 60, to compute, e.g., a time of flight from theposition in the droplet path above the ignition site 28 to the ignitionsite 28, as detected, e.g., by the other two of the three detectors 262,262 a and 262 b.

Due to, e.g., limitations in the time of response, sensitivity ofresponse or the like, of the detectors, e.g., 262, 262 a and 262 b, theabove referenced tracking system may not be responsive enough or provideenough data or data that can be processed quickly enough for purposes ofaccomplishing some or all of the desired functionalities of the targettracking system 42 according to aspects of embodiments of the presentinvention, at least on a droplet by droplet basis.

One of the imaging devices and detector 256, 256 a and 256 b and 262,262 a and 262 b may be formed with, e.g., an elongated cylindrical lensto form, e.g., a planar detection plane above the plane of the ignitionsite, e.g., as viewed in FIG. 4A, e.g., to detect the passage of thedroplet target 94 through the plane. In such an event, a systemillustrated schematically in FIG. 5 may be used to supplement or replacesome or all aspects of the target tracking system described in relationto FIGS. 4A and B.

The laser beams 256, 256 a and 256 b may be generated by differentlasers than a HeNe, or, e.g., they may be, e.g., frequency doubled andadded to obtain, e.g., harmonics in order to be able at the detectors262, 262 a and 262 b to discriminate between the detected image light,e.g., by using photo-diodes sensitive only to the specific frequency to,e.g., eliminate cross-illumination of the detectors 262, 262 a and 262b.

In FIG. 5 there is shown schematically a possible high resolution targettracking system 42. FIG. 5 illustrates schematically the intersection inthe vicinity of the ignition site the fields of view 270 a, 272 a and274 a of, e.g., three imaging cameras, e.g., 70, 72 and 74 shownillustratively in FIG. 1, except for the modification that in FIG. 5 allof the camera fields of view intersect each other and may, e.g., allintersect at the ignition site 28. As in the example shown in FIG. 5,each of the fields of view may be mutually orthogonal to the others.FIG. 5 also illustrates one of the fields of view, e.g., 270 extendingback to, e.g., a square array of detector pixels 270, e.g., in animaging camera 72, which may, e.g., be a digital camera, e.g., employinga square array 270 of pixels, e.g., each formed by charge coupleddevices or CMOS imaging integrated circuits or a single chip CCD or CMOSimager or the like as are well known in the digital camera art.

It will be understood, that the imaging cameras 70, 72 and 74 may, e.g.,be supplemented with a plane crossing detector as discussed above inregard to FIGS. 4A and B or in regard to FIG. 9, or another camera aimedto have a field of view above the ignition point 28 to, e.g., get timeof flight information and positioning information for above the ignitionsite 28, e.g., for calculations of, e.g., trajectory of the targetdroplet, e.g., near the ignition site 28.

With such an apparatus, e.g., one may be able to form an image of thedroplet 94, e.g., formed by a generally circular group of pixels fromthe array 270 and using suitable image processing software track the“blob” image of the droplet across the array. One skilled in the art ofimage processing and object tracking will understand that such trackingacross three intersecting fields of view, e.g., 270 a, 272 a and 274 acan provide tracking of the droplet 94 before it reaches the ignitionsite 28, and provide, e.g., information from which an error signal canbe generated, e.g., by the feedback controller 62 indicating a positionerror between the actual position of the target droplet and the targetignition site 28, which may for the given droplet be based on an aimingpoint for the laser beam 154 for that particular target droplet 94,which may or may not be at some preselected desired ignition point,e.g., at the collector focus, e.g., due to feedback controls, e.g., tothe laser aiming system 68, as explained in this application.

It will also be understood that only two cameras could be aimed at theignition point 28. Further the sensitivity of the cameras, e.g., 70, 72and 74 could be such that only one pixel at a time is illuminated by theimage of the target droplet and/or that the fields of view 270 a, 272 aand 274 a could be very high resolution (low pixel pitch) in order tosee the target droplet, and also relatively small in field of view,thus, e.g., decreasing the ability to significantly track the flight ofthe target droplet, e.g., in the vicinity of the ignition site 28,making the use of detection of the target droplet above the ignitionsite more important to the overall functioning of the target trackingsystem 42.

The output of the target tracking system 42 is desired to be informationabout the target droplet 94, especially at or near the ignition site 28,from which, e.g., the target tracking feedback control system 62 canprovide information to the main controller 60 that indicates, e.g., atarget droplet position and trajectory at some time prior to reachingthe ignition site 28, and e.g., a predicted time of arrival of thedetected target droplet 94 at the ignition site 28 and the location atthat arrival time in relation to the currently selected aim point forthe laser beam 154, so that, e.g., the currently selected aim point maybe moved to the predicted point. Also needed may be, e.g., the actualobservation of the target droplet arrival at the ignition site and,e.g., the interaction of the laser beam 154 and the particular targetdroplet 98 at the ignition site 28, and perhaps, also, imaging of anydebris departing the ignition site 28. All of the above may then be usedby the system, e.g., to generate feedback to, e.g., the main controller60, such that the main controller 60 may serve to generate controlsignals to, e.g., modify the target droplet delivery by the targetformation/delivery system 24 and/or the positioning of the aim point ofthe laser beam 154, e.g., by controlling the focusing optics 156, andalso, e.g., the timing of the firing of the laser beam 154 at the aimpoint 28, e.g., by triggering the initial charging of the pulse powersystem resonant chargers in the magnetic reactor-switched pulsecompression and timing circuits 50, 52, for, e.g., the MO and PA laserchambers, and the triggering of the respective firing of the MO and PAchambers, e.g., to deliver the pulse of laser light in beam 154 at theignition site 28 timed to the arrival also of the target droplet 94. Thetarget droplet 94 and the pulse of laser light 156 must arrive at theparticularly designated ignition site 28 for that droplet 94 and thatbeam 156, with a combined position error of less than about 10 μm, sothat the focused pulse 156 of laser light irradiates the entire targetdroplet 94 without any of the droplet being outside of a spatialdistribution of the energy in the pulse 156 that is below some selectedlevel of intensity, in order to avoid, e.g., chunks of metallic debristhat will, e.g., pit or coat and optically degrade and/or damage, e.g.,reflective surfaces in the EUV light source system 20. The system 20may, e.g., need to provide a 50 microsecond lead time for triggering theproper firing of the laser 22, particularly if it is a MOPAconfiguration, e.g., a KrF MOPA, with an accuracy of about 1microsecond, once every 250 microseconds, e.g., at a 4 KHz repetitionrate and once every 100 microseconds for a 10 kHz pulse repetition rate.The droplets 94 will be arriving, e.g., a speed of about 20 meters persecond and separated by about 1 mm at a 6 Khz pulse repetition rate.

Since it takes some finite time to generate the laser pulse beam 154from some occurrence of a triggering signal of some kind, and due to thelength of that time, and other factors, e.g., computing time, trackingdevice and circuitry time, etc. current technologies may not allow forsuch triggering on a droplet by droplet basis, particularly at higherrepetition rates, e.g., at or above 4 kHz. In such event, the detectionssystem 42 and the feedback controllers, e.g., 60, 62 may have to rely,e.g., on timing and position control and the like, e.g., based uponaveraging, e.g., droplet positioning and timing information over aseries of successive droplets, e.g., the last x number of droplets, andmake assumptions about the succeeding droplets continuing to be within,e.g., some relatively slowly varying deviation from the averagedpositions so determined. In such a case, the system may still require,e.g., position/timing detection of a given droplet above the ignitionsite, e.g., for firing control of the laser system 22.

Turning now to FIG. 6, there are shown schematically a number of otheraspects of embodiments of the present invention, e.g., featuresincluding cold fingers 280, pressure interface including a pressureshield 290 and a vacuum pump 300. The cold fingers 280, only some ofwhich are shown, may be comprised of, e.g., magnesium coated copperplates, that may be curved, as shown and may be separated by largerdistances more toward the intermediate focus 40 can be water cooled,e.g., with a heat exchanger system (not shown) and also, e.g., withmicro-channels inside of the cold fingers 280 (also not shown), e.g., asis done by fusion bonding two pieces together to form each cold finger280, e.g., as illustrated in co-pending U.S. patent application Ser. No.10/607,407, entitled METHOD AND APPARATUS FOR COOLING MAGNETIC CIRCUITELEMENTS, filed on Jun. 25, 2003, Attorney docket No. 2003-0051-01,assigned to the common assignee of the present application, thedisclosure of which is hereby incorporated by reference. These coldfingers, which as partly illustrated schematically in FIG. 6 may extendthroughout the vessel 26 except in the cone of the EUV light focused tothe intermediate focus all the way or part of the way back to anintermediate wall 282 in the vessel 26. They serve to plate out sourceatoms that were formed in the plasma or carried with the plasma as itexpands from the buffer gas, e.g., argon in the vessel 26, so that theseatoms to not plate out on optical surfaces in the EUV light source.

Also shown in FIG. 6 is a possible interface to the exterior of the EUVlight source, e.g., outside of the intermediate focus, which may bemaintained in a vacuum to limit absorption of EUV light. However, thevacuum in the other portion of the chamber where the EUV light isproduced may, for various reasons, need to be maintained at a highervacuum. The interface may comprise, e.g., an intervening wall 282 and apressure shield, i.e., a differential pumping trap 290, which may bedesigned, e.g., to permit the transmittance of the EUV beam to theintermediate focus while maintaining a pressure drop from the portion ofthe vessel 26 on the one side of the intervening wall 282 that is underpressure, to the other side being maintained at or near a the vacuum byof the enclosure beyond the intermediate focus, e.g., a vacuum pump 300.The differential pressure trap may be constructed similarly to a form ofdebris shield disclosed in co-pending U.S. patent application Ser. No.10/742,233, filed on Dec. 18, 2003, entitled DISCHARGE PRODUCED PLASMAEUV LIGHT SOURCE, Attorney Docket No. 2003-0099-01 assigned to thecommon assignee of the present application, the disclosure of which ishereby incorporated by reference. This may be constructed to havechannels for the passage of focused EUV light to the intermediate focus,but of sufficiently small size for each channel that the pressure dropacross the differential pumping trap can be sustained. To this effect,the differential pumping trap 290 may also be constructed, e.g., byusing a section of a sphere of material, e.g., ceramic material and,e.g., focusing a laser through a lens and a meshed screen to drill,e.g., focused passageways through the portion of the sphere to allow theEUV light through, while sustaining the pressure drop, also as disclosedin the above referenced U.S. patent application Ser. No. 10/742,233.

Turning now to FIG. 6 there is shown in more detail aspects of afeedback and control system according to an embodiment of the presentinvention.

With a full 2π steradian multi-layer collector, the required electricalinput power to the laser for generating the laser to create the plasmacan be reduced by 25% due to increasing the geometric collection areafrom 5 steradians to 2π steradians. For example, for a KrF excimer basedLPP source assuming, e.g., 2.0% laser-to-EUV conversion (based on, e.g.,double efficiency with short wavelength), 4% electrical-to-laserconversion, 2π steradian collection and the same EUV transmission as theTRW/CEO system, the resulting electrical power is 227,272 W, whichcompares well to an alternative approach using discharge produced plasma(“DPP”). For example, (2.0%/1.0%)·(4.0%/3.0%)·(2π str/5 str)=3.3 leadsto this amount of improvement over, e.g., the values shown in thepresent published TRW/CEO LPP results.

With such possible CE results, one can also estimate the laser powerrequired to meet, e.g., 100 W of EUV light power at the intermediatefocus as follows: Single Second elliptical Second Spherical SphericalCollector Collector¹ Collector¹ In-band Power 100 W 29 W 60 W at IFBuffer gas 0.90 .90 0.90 transmission Power reflected 111 W 32 W 67 Wfrom collector Ave. reflectivity .50 0.60 * 0.50 = 0.30² 0.60 * 0.50 =0.30² of collector Power incident 222 W 107 W 222 W on collectorFraction of 2π sr 0.795 0.795 0.795 subtended by collector (5 srcollector) Power emitted 279 W 135 W 279 W into 2π sr In-band CE 0.0310.015³ 0.031⁴ for lithium Input laser power 9,017 W 9,017 W 9,017 WNotes:¹The calculations for the second spherical mirror (columns 2 and 3)start with the input laser power and work upward to see what additionalEUV power would be available if a second spherical mirror was added.²Since the radiation reflected by the second spherical mirror must thenbounce off the primary elliptical mirror, the effective reflectivity isthe product of the two mirrors. The second spherical mirror, since allrays are reflected at normal incidence, was a higher assumed averagereflectivity of 60%.³This column assumes that only half as much radiation is emitted in the“backward” direction as compared to the direction toward the incidentlaser beam.⁴This column assumes that the emission in the “backward” direction isequal to that in the direction toward the incident laser beam.

Summing the first and second columns one gets 129 W of EUV at the IF for9,017 W of laser power, which means, e.g., that one only needs 6,989 Wof laser power. Doing the same for columns 1 and 3, leads to aconclusion of a requirement for only 5,636 W of laser power to reach 100W at the IF. Its still a lot of laser power, but not in the20,000-40,000 W range, e.g., as described in the results from, e.g.,TRW. There is presented a possible economic trade-off between the secondspherical mirror and otherwise increased laser power.

Applicants have considered the situation for, e.g., a lithium target/KrFdriven LPP, and initially concluded that essentially all of theradiation passed on to the intermediate focus point (simulated in anexperiment by a photodiode detector) is either in-band 13.5 nm radiationor UV-Vis radiation. There is no out-of-band EUV, thanks to the use of amulti-layer mirror (“MLM”) collector arrangement (also simulated by justa flat MLM). There also appears to be no significant radiation in theregion between 40-130 nm. In addition, the conversion efficiency intoin-band 13.5 nm radiation appeared to be 4.3 times higher than into theUV-Vis region. However, the requirement seems to be for only 1/100th(1%) as much energy in the 130-400 nm range as the in-band at 13.5 nm,whereas according to applicants' initial experimental measurements theUV-Vis range contains 22% as much energy as the in-band 13.5 nmradiation. However a large portion of this appears to be UV-Vis range tobe a strong red line at 670 from neutral lithium, along with other lightin the 120 nm-9000 nm range. In addition what was experimentallymeasured by applicants was radiation from all points around the EUVsource point, whereas the true source, e.g., in the configurationcontemplated by applicants would have an elliptical imaging mirror andan aperture at the intermediate focus, the latter of which can, e.g.,block all radiation from regions away from the EUV source point.

For all LPP systems, e.g., with an MLM primary collector, the 10-40 nmrange can be dealt with by the narrow-band reflectivity of the MLM,unlike, e.g., DPP systems, with a grazing incidence collector, where,e.g., all EUV radiation is re-imaged to the intermediate focus and thusthis range may be a problem in regard to out of band radiation, without,e.g., a spectral filter, which can, e.g., decrease CE further thanoperating a system without one, especially for tin and xenon plasmasource emission element materials. This may not be true, however forlithium. The same can be said for the 40-130 nm range, because the MLMprimary collector in the LPP system also exhibits low reflectivity inthis region, but the grazing incidence collector in a DPP could haverelativity high reflectivity in the region 40-130 nm.

Between 130 nm and 400 nm, the MLM primary collector is just asreflective as for in-band 13.5 nm radiation, and thus, e.g., the sourcemust emit 100 times less energy in this wavelength range as in-bandenergy. This restriction is primarily due to the fact that most EUVphotoresists are sensitive to this wavelength range as well as 13.5 nm.Though the MLM's in the exposure tool reflect the 400-800 nm range justas well as in-band 13.5 nm, the photoresist is not sensitive and thusonly mirror heating is an issue. Thus, the system can tolerate an equalamount in this range as in-band at 13.5 nm. Since MLM's are highlyreflective for wavelengths, above 800 nm, but the photoresists are notsensitive to these wavelengths, it would appear that the range above 800nm would have the same restrictions as the 400-800 nm range. In terms ofa YAG-based LPP 1064 nm is included in this last range, and, therefore,2% conversion efficiency into in-band 13.5 nm can be accompanied by onlyhaving 0.001% scattering of the pump laser if that were a requirementfor the above 800 nm range.

It is apparent from the above, why CE and in-band CE are so important.

Applicants' experiments have given the following results for solid tinand lithium targets for comparison purposes: Tin Lithium Laser inputenergy 165 mj 165 mj Total 4π emission from plasma 80-88% 15-20% as apercent of input energy (all wavelengths) UV-Visible 4π emission fromplasma    3%   0.8% as a percent of input energy (150 nm to 9000 nm) EUV4π emission from plasma 20-25%  5-7% as a percent of input energy (Zrfilter band, 6.5 nm to 17 nm)

Turning now to FIGS. 7A-C there is shown an apparatus and method forelectrostatically confining a, plasma, e.g., a laser produced plasmaaccording to an aspect of an embodiment of the present invention. Asshown in FIG. 7A, a thin needle 310 may be provided extending into thevicinity of the ignition site 28. The needle 310 is shown in FIGS. 7A-Cto extend from a direction opposite to that of the incoming pulse oflaser light 154 passing through the laser beam 154 focusing optic 156,but those skilled in the art will appreciate that this particularorientation is exemplary only and the needle can extend to theillustrated proximity to the ignition site from other orientations aswell.

The needle 310 may, e.g., be provided with a source of high voltage,e.g., negative high voltage, 312 and be controlled, e.g., by the overallsystem controller 60 or, e.g., as part of the laser triggering control,to coordinate the provision of a high negative voltage pulse to thearrival of a target droplet 94 and the laser pulse 154 to the ignitionsite 28, such that at or just after ignition of the target droplet atthe ignition site by irradiation from the laser beam 154, anelectrostatic field 314 is formed to confine or assist in confining theplasma 316 produced by the irradiation of the target droplet 94. Thismay have several beneficial results, e.g., limiting or essentiallyeliminating plasma produced debris from reaching, e.g., the collectoroptics, maintaining the plasma sufficiently small to increase ionizationof the material of the target droplet thus improving the CE, i.e.,helping to maintain plasma density of the plasma 316 all during theirradiation by the laser pulse 154.

The voltage may be, about, e.g., 1000, which should be sufficient forthe creation of an electric field capable of keeping ions of an energyof up to about 1 keV, which is in the range of the plasma ions. Inaddition as the field begins to form, e.g., by the introduction ofelectrons with negative charge into the needle 310, the positive chargesin the plasma due to ionization of the target material may be attractedto the needle to a large enough extent to keep the electrostatic field314 from ever forming or relatively quickly smothering the electrostaticfield 314. To counteract this, applicants propose to provide the voltagesupply 312 with a relatively large capacitor, e.g., a bank ofcapacitors, e.g., in parallel to combine the capacitance to, e.g., e.g.,100 μF or even larger as is possible, so as to relatively quickly dumpinto the needle 310 enough negative charge to prevent the positivelycharge ions forming in the plasma from preventing the electrostaticfield from performing the intended confinement of the plasma at andafter ignition.

The above description of aspects of an embodiment of the presentinvention are illustrative only and the claims should not be consideredto be limited to the disclosed embodiment(s). Many changes andmodification may be made to the disclosed embodiments without departingfrom the scope and intent of the appended claims. FIG. 8A showsschematically a magnetic apparatus and method to confine the plasma inthe vicinity of the ignition site 28 after ignition. FIG. 8A shows themagnetic field 320 set up by, e.g., a pair of bar magnets, 326, 328.FIG. 8B shows magnetic field lines 320 schematically illustrating themagnetic field of a ring magnet 322, which serve to confine a plasmaformed at the ignition site 28 when a target is irradiated by a laserbean, e.g., 154 shown in FIG. 8C. FIG. 8B also shows a the use ofcooling for the permanent magnet, e.g., a neodymium iron boron magnet ora samarium cobalt magnet, both manufactured, e.g., by DexterCorporation, under the name of Permag type NdFeB40, and Permag typeSmCo22, e.g., a in the form of a ring magnet 322, e.g., using coolingcoils 324, e.g., containing flowing cooling fluid, e.g., water. FIG. 8Cshows schematically the field 320 set up by a quadrapole arrangement329.

Turning now to FIGS. 8A-G there is shown schematically aspects of anembodiment of the present invention. The magnetic field 320 may also beset up by pulsed current, e.g., as shown in the embodiments of FIGS.8D-G. In FIG. 8D there is shown a schematic view of the electricalequivalent of the ring magnet of FIG. 8B, e.g., with a magnetic field320 set up by pulsed current flowing, e.g., through coils of wireindicated by current flowing into the plane of the paper at 330 and outof the plane of the paper at 331. Similarly, FIG. 8E shows an embodimentwhere a generally bottle shaped magnetic field is set up by distributingthe coils along the length of the magnetic field generator so that thereare more windings at either end. Similarly in FIG. 8E, this same shapedfield 320 can be established, e.g., by alternating the direction ofcurrent flow in the coils from one end to the other, i.e., havingcurrent flow propagate in one direction through the coil at one end andin the other at the other end, and for a similar purpose a generallyspherically shaped coil arrangement can be used, e.g., as shownschematically in FIG. 8G.

By applying a magnetic field in the neighborhood of the ignition site,e.g., of about 1 Tesla in a fashion to create a field in the region ofthe laser produced plasma the plasma may be at least partially confined,e.g., because plasma expansion can be slowed down, at least in somedirections, depending on the magnetic field shape and strength in thevicinity of the respective part of the plasma. This assist inconfinement can have several benefits, especially for a moving target ofthe laser irradiation. For example, the radiating ions will then tend toundergo more radiation cycles and, therefore, emit more radiation. Morelaser energy can then be converted to radiation rather than, e.g., ionexpansion energy resulting in a higher CE of incident laser energy intoEUV light.

The magnetic field and the mechanism 318 used to create it can beconveniently arranged to encompass within an appropriate part of thefield the ignition site and to allow the target, e.g., a droplet and theirradiation laser beam access to the ignition site. The laser plasmaregion formed when the laser beam irradiates and ignites the targetdroplet being in the magnetic field according to an embodiment of thepresent invention. While typically the field may be about 1 Tesla arange of between about 0.2 and 10 Tesla is contemplated by applicants.The field may be generated using the above noted permanent magnets or inthe above described pulsed fashion using, e.g., a high (kilo-ampere)pulsed current through a conducting coil as discussed above. Such apulse generated magnetic field may be generated, e.g., on a microsecondscale of time and be made to remain essentially constant throughout thetime of the irradiation of the target droplet by the incoming laserpulse, e.g., on the order of e.g., several tens of ns. During that time,e.g., the plasma expansion across magnetic field lines is slowed andmotion along the field lines is not substantially slowed, the net effectperhaps inducing plasma instabilities which are outweighed, e.g., byincreases in CE.

Higher magnetic pressure, e.g., increases the collision frequency in theplasma, which can cause, e.g., a smaller volume hotter plasma thanwithout the magnetic field. Consequently more radiation in the EUV andotherwise is emitted according to the target material and plasmacharacteristics. One possible embodiment is to use a transversemagnetic, field, e.g., as shown in FIG. 8A. Another is to used a strongring magnet or magnetic coil around and near the ignition site which cangenerate, e.g., magnetic field lines along the target dropletpropagation path and, e.g., lead to axial confinement in the vicinity ofthe ignition site 28. A preferred embodiment is a configuration, e.g.,as shown in FIG. 8C, E, F and G, e.g., in which is formed a magnetictrap, e.g., where target ions with traverse (to the target droplet path,illustrated, e.g., in FIG. 8C) are confined.

According to an aspect of an embodiment of the present invention, themagnetic field creation mechanism, e.g., the poles of a permanentmagnet, e.g., 326, 328 may have to be relatively close to the LPP, with,e.g., about a 10 mm gap between poles. It may be, e.g., difficult, ifnot impossible, to create a high enough magnetic field strength, e.g.,through long distances between the poles. This need for a close approachcomponent, can, e.g., detract from one of the greatest advantages of theLPP, an absence of electrode erosion, or in this case, e.g., permanentmagnet erosion. Applicants have examined, e.g., the nozzle distance forassured positional stability which appears to be on the order of about50 mm. To mitigate erosion problems with these components, e.g., nozzleand permanent magnets, applicants propose to, e.g., accept erosion butto cause the erosion to be of an acceptable material, e.g., by coatingall close-approach elements with, e.g., molybdenum or ruthenium. In thismanner, e.g., the eroded material from these components, which mightfall on the collector mirror, will not rapidly degrade the mirrorreflectivity. Also, e.g., these two materials are expected to have highresistance to sputter by lithium ions.

Turning now to FIG. 9 there is shown a block diagram of an aspect of anembodiment of the present invention regarding feedback and control,providing, e.g., six degrees of feedback and control, i.e., three axiscontrol for steering the target droplets and three axis control forsteering the laser. It will be understood that the laser beam may besteered, e.g., utilizing beam pointing and positioning controls, e.g.,those used in laser bean delivery units, e.g., as described inco-pending application Ser. No. 10/739,961, GAS DISCHARGE LASER LIGHTSOURCE BEAM DELIVERY UNIT, filed on Dec. 17, 2003, Attorney Docket No.2003-0082-01, Ser. No. 10/712,688, LASER LITHOGRAPHY SOURCE WITH BEAMDELIVERY, filed on Nov. 12, 2003, Attorney docket No. 2002-0039-06, and10/425,361, LITHOGRAPHY LASER WITH BEAM DELIVERY AND BEAM POINTINGCONTROL, filed on Apr. 29, 2003, Attorney Docket No. 2003-0040-01. FIG.9 is an illustration schematically and in block diagram format variouscontrol loops employed according to aspects of an embodiment of thepresent invention. There are, e.g., several different actuators that canbe utilized in an EUV light source according to aspects of an embodimentof the present invention that can, e.g., be used in a control systemconfiguration. At repetition rates of, e.g., 10 KHz, the droplets willbe arriving at one every 100 microseconds, and traveling at about 10-30m/s, so that the laser beam will have to be time to irradiate a desiredtarget ignition point at the same rate. The laser beam may be focused tobe slightly larger than the target droplet, which droplet may be about10-50 μm in diameter, with some degree of aiming tolerance, e.g., ±10μm, however the higher the degree of error tolerance embodied in thebeam focus size, the lower the power irradiating the droplet target,which decreases in a square function. The droplet, to the extent itcontinues to move at all during the irradiation period, however, willonly move several tenths of a nanometer.

One set of actuators may include, e.g., x and y axis magnetic fields,which may be generated, e.g., by sets of electrodes or coils (notshown), e.g., contained in a target steering and acceleration mechanism360 used to create magnetic fields which can, e.g., steer a target,e.g., a lithium droplet 94 to the correct intersection point with thelaser beam (the desired ignition point). This could be implemented,e.g., with one set of electrodes (not shown), but other implementationsmight use multiple sets of these to give better trajectory control. Inaddition there may be a set of electrodes (not shown) that create, e.g.,a z axis magnetic field, e.g., used to accelerate the target, e.g., alithium droplet along the z-axis. It will be understood that this aimingand acceleration function may also be implemented with electric coilsarranged in the path of the droplet to deflect the droplet, e.g.,towards or away from a respective coil and/or accelerate along thelength of a coil. The acceleration and deflection may be magnetic ininitiation or electrostatic, as is understood in the art. Targetsteering may employ techniques such as discussed in M. Orme et al.,“Charged molten metal droplet deposition as a direct write technology,”MRS Spring Meeting, San Francisco (2001) and Orme et al., “Electrostaticcharging and deflection of nonconventional droplet streams formed fromcapillary stream breakup,” Physics of Fluids, Vol. 12, No. 9 (September2000), pp. 2224-2235, the disclosures of which are hereby incorporatedby reference.

The droplets may, e.g., be charged, e.g., by placing a charge ringaround the nozzle 220 of the target delivery system 24. Since thedroplets are small, the charge distribution over the droplet may beconsidered to be relatively uniform, however charge will tend toaccumulate at points of higher curvature, so that droplet distortions,if any may alter the charge distribution. To account for this, accordingto an aspect of an embodiment of the present invention, the droplets maybe passed through a differential charge analyzer (not shown)intermediate the target delivery system 24 and the steering andacceleration mechanism 360, which may comprise, e.g., a pair ofelectrodes that deflect the droplet in opposite directions, which thedifferential in deflection being a measure of charge non-uniformity.This differential in displacement may be detected using detectors (notshown) as discussed in the present application. The amount of chargenon-uniformity may be used by the system 350 to control the x and ydeflection of the droplet in the steering and acceleration mechanism 360and z-axis acceleration as well.

According to aspects of an embodiment of the present invention the laserbeam system 22, the two chamber excimer laser source containing an MO 44and a PA 48, could be operated, e.g., in voltage control mode, with thecontrol system 350 controller 362 can, e.g., provide voltage commands tothe MO and PA acting as actuators, to regulate the energy out of the EUVsource 20. Alternatively, the MOPA 22 could be operated, e.g., in aconstant energy mode, in which case, the control system could provideenergy commands to the MOPA that could, e.g., be actuators used toregulate the energy out of the EUV source 20. The control system couldprovide a laser trigger signal to the laser system 22 to act as anactuator providing the laser pulse, in order, e.g., to control thearrival time of the laser pulses at the desired droplet ignition site.As a further alternative the laser may be controlled from the controller362 by sending, e.g., firing control signals directly to the MOPA TEMutilized in laser timing control systems in MOPA laser products sold byapplicants' assignee, e.g., XLA laser models. The voltage control andoutput energy control signals may be commanded in unison or separately.

Along with and/or in addition to the tracking an position detectionsensors discussed above, according to aspects of an embodiment of thepresent invention, e.g., several different sensors can be made availablein the EUV source 20 control system 350 illustrated in FIG. 9, e.g., tobe used in a control configuration. By way of example, sets of photocells, e.g., a first x axis photocell array 364 and a second x axisphotocell array 365 and a first y axis photocell array 366 and a secondy axis photocell array 367, e.g., may be arranged perpendicular to thedroplet path and may be used, e.g., to determine the droplet trajectory,e.g., by determining x and y position of the droplet, e.g., as comparedto a predicted x and y axis position at the point of the x and yphotocell arrays 364, 366 after leaving the target delivery system 24,or, alternatively, by detecting the x and y positions of the droplet atthe arrays 364, 366 and at the arrays 365, 367, and comparing the two,knowing the distance between the two. Neither the x and y photo-arrays364, 366 not the x and y photocell arrays 365, 367 need to be co-planar,but they may conveniently be so. Another alternative is to userespective arrays to determine droplet arrival time at the sensor 364,365, 366 and/or 367, which may be used, e.g., as a z plane crossingindication. These detectors 364-367 could be implemented, e.g., by sideimaging lasers, as explained above. They may be read, e.g., once perdroplet passage, and can provide as an output, e.g., an integrated valueof the light detected by each photocell in the array, e.g., over aselected time period, with, e.g., the peak of the inverse of theseillumination intensities indicating a location on the photodiode array,which may only be twenty or so photodiodes (pixels) in length,indicative of the position of the center of the droplet in the axisalong which the photodiode array is oriented. It will be understood thatthis peak in the time domain, or perhaps the leading or trailing edge ofthe spectrum of the integrated signals from the photodiode array mayalso indicate a z plane crossing time.

In addition, alternatively, e.g., one or more z axis lasers, e.g., aHeNe laser as discussed above can be used, e.g., to measure the timewhen a droplet crosses the beam, e.g., 370, 372, 374 or 376 which maycomprise a planar beam oriented in the plane of the z-axis, i.e., in thedirection of transverse to travel of the target, e.g., a droplet oflithium, from the target dispensing system 24 to the ignition site 28.There may, according to aspects of an embodiment of the presentinvention be multiple such z-axis detection planes, e.g., 370, 372, 374and 376, e.g., used to control the timing of pulses applied to themagnetic fields, e.g., contained in the droplet steering andacceleration mechanism 360. The magnetic fields could, e.g., be pulsed afixed interval after the droplet crosses the plane of a respective beam,e.g., 370, 372. Additionally, there could be beams, e.g., 374, 376positioned closer to the desired ignition point. Crossing this beam 374could be used, e.g., to cause the lasers system 22 to be triggered at aprogrammed interval or intervals after beam crossing. The crossings of aplurality of z axis laser planes, e.g., 370, 372 and 374 can also beused, e.g., to determine droplet speed. Detectors 364 a, 366 a and 365a, 367 a may be used, e.g., for trajectory and/or speed detection belowthe steering and acceleration mechanism 360. It will also be understoodthat, e.g., only one plane crossing may need to be used for lasertriggering and, e.g., the laser control system 64 itself can be used tocontrol the timing between the MO and the PA to effectively deliver alaser pulse to the ignition site 28 timed to arrive concurrently withthe target 94, at some defined time interval after such trigger signal,as is well understood in the art of MOPA laser timing control systems.

According to aspects of an embodiment of the present invention, e.g.,when a droplet moves through the plane of an x or y axis photocellarray, e.g., 364, 366, the voltage cell will produce a voltage patternon the outputs of the photocells, e.g., indicating a level of lightintensity at each individual photocell (not shown) and this informationmay, e.g., be provided to the controller 362. An algorithm can then beused by the controller 362 to turn this information into a dropletposition, e.g., in the x and y planes as noted above. Given state of theart available photocell arrays of affordable cost and acceptableresolution, the algorithm will have to, e.g., achieve measurementprecision higher than the pitch of the photocells (not shown) in thearray, e.g., 364, 366, as is understood in the field of utilization ofsuch photo-diode arrays in the field of laser wavelength and bandwidthdetection. Additionally, according to aspects of an embodiment of thepresent invention the algorithm may also be able to measure droplet sizeand droplet deformation, e.g., by utilizing the outputs of the x, ydetectors, e.g., 364, 366, e.g., to detect droplet width in two axes.

According to an aspect of an embodiment of the present invention theremay be, e.g., two possible stages of position control. In the firststage, x and y photocell arrays 364, 366, 365, 367, may be used, e.g.,to determine the z plane position of the droplet prior to entering the xand y axis magnetic field electrodes in the droplet steering andacceleration mechanism 360, along with the droplet trajectory. Thisinformation may then be used by the control system 362 to, e.g., adjustthe sizes of the fields applied at the electrodes (not shown), e.g., forthe current droplet and perhaps also for the next subsequent droplet. Ina second stage, e.g., x and y photocell arrays 364 a, 366 a may be used,e.g., to determine the position of the droplet after it has passedthrough the x and y axis magnetic fields in the droplet steering andacceleration mechanism, and, e.g., just prior to intersection with thelaser beam at the desired ignition site. This information may, e.g., beused to adjust the x and y magnetic fields, e.g., for successive shots,e.g., to adjust for any position error in the droplet previouslyarriving at the ignition site.

The z-axis laser plane detectors 374, 376 or the x and y axis photocellarrays, 364 a and 366 a, and 365 a and 367 a, or a combination thereofmay also be used, e.g., to determine the droplet speed and trajectory,e.g., after leaving the Z-axis magnetic field in the droplet steeringand acceleration mechanism 360. This can then be used, e.g., to adjustthe z-axis magnetic fields for successive targets, e.g., based upondetected target position/speed error for a droplet in a prior shotarriving at the desired ignition site. In addition, e.g., a pairdetectors (not shown), e.g., only including a single photo-diode element(pixel) may be illuminated by a respective pair of beans one passingthrough a point at or very near the desired target ignition site and onejust above that, e.g., to detect target speed as close to the desiredtarget ignition site as possible, e.g., by the leading edge of thedroplet blocking each of these two respective detectors (not shown).This may be used, e.g., to indicate speed changes occurring in thetarget droplets at or very near the desired target ignition site, e.g.,due to influences of the creation of a plasma by ignition of a priordroplet target, magnetic field influences or the like.

A dither may also, e.g., be is applied to the energy setpoint for, e.g.,the excimer target irradiating laser. The dither signal can be eitherrandom or periodic. The dither may, e .g., be correlated with the EUVoutput energy to determine the sensitivity of EUV output to the energyoutput of the plasma forming laser. This information may be used, e.g.,to scale the commands in the plasma forming laser energy control loop tokeep the loop gain constant.

According to an aspect of an embodiment of the present invention theremay be two laser systems 22 each providing a laser pulse to the desiredignition site, time to arrive simultaneously, in which event, arrivaltime of each laser pulse to a point just prior to droplet intersectionmay be measured. This value may, e.g., be used to adjust the triggertime of each laser relative to the droplet crossing the final z-axislaser beam plane. Also in the case of the use of two lasers 22,independent dither signals may be applied to the trigger times of eachof the excimer lasers 22. These dithers may be correlated with the EUVoutput, e.g., in order to determine the sensitivity of the EUV energy tothe trigger time of each excimer laser. The trigger time of each excimerlaser may then independently adjusted to drive the sensitivity to zeroand thus maximize EUV efficiency.

According to another aspect of an embodiment of the present inventionthe above mentioned sensors may be used to determine a target positionand trajectory and to predict a desired ignition site in the possiblepaths of the laser pulse and provide feedback to control either theaiming of the laser 22 into the laser positioning and focusing optics(not shown) or the aiming of the laser positioning and focusing optics(not shown) or utilizing beam pointing as discussed above, for purposesof causing an intersection and irradiation of the respective droplet bythe laser beam pulse at a predicted desired intersection point(predicted desired ignition site), which, e.g., may be different from aprior ignition site, but still within an acceptable distance from thefocus of the collector 30 to not significantly detract from thecollected EUV light. This may also be done in a relatively slow feedbackloop, i.e., not on a shot by shot basis, to correct for systemindications of a slow drift of the average arrival time and position atthe target ignition site. Thus over time, e.g., due to changes in theoperating environment within the vessel, e.g., buffer gas pressure, thedesired target ignition site may move slightly, staying within, e.g.,about a ±10 μm position error from the focus of the collector 30 andstill generate EUV light at acceptable levels. The system just describedmay be used to detect this change over time and to redirect the focus ofthe laser to the new desired target ignition point, assuming that thesteering mechanism due to environmental changes is not able on averageto direct the target droplets to the original target ignition point,e.g., at the focus of the collector 30.

Target delivery may also be accomplished utilizing techniques such asthose disclosed in co-pending U.S. application Ser. No. 10/409,254,EXTREME ULTRAVIOLET LIGHT SOURCE, filed on Apr. 8, 2003, Attorney DocketNo. 2002-0030-01, the disclosure of which is hereby incorporated byreference.

Turning now to FIG. 10 there is shown aspects of an embodiment of thepresent invention comprising an input window 380 formed in a wall of thechamber vessel 26 and through which the laser bean 146 enters to reachthe ignition point 28. The window 380 may be, e.g., heated to, e.g.,remove, e.g., by evaporation debris that plates onto the window, e.g.,lithium, tin or xenon atoms from the plasma. The window 380 may beheated, e.g., by a heating element, e.g., by use of an external heatingfixture attached to the metal body of the window 380 mounting flange ora heat lamp 382, e.g., an infrared heat lamp, which may, e.g., bereflected onto the window 380 by a mirror 384. The 384, as is shown inFIG. 10, may, e.g., be facing away from the laser plasma in order toavoid that mirror 384 surface being in a direct line of sight to theplasma. This can, e.g., prevent particle impact from the plasma ontothis mirror 384 reflective surface.

Those skilled in the art will appreciate that many modifications andchanges may be made to the above described aspects of embodiments of thepresent invention and the appended claims should not be interpreted tobe limited only to the disclosed embodiments, but to include suchembodiments and equivalents thereof.

1. An EUV light source comprising: a pulsed laser providing laser pulses at a selected pulse repetition rate focused at a desired target ignition site; a target formation system providing discrete targets at a selected interval coordinated with the laser pulse repetition rate; a target steering system intermediate the target formation system and the desired target ignition site; a target tracking system providing information about the movement of target between the target formation system and the target steering system, enabling the target steering system to direct the target to the desired target ignition site.
 2. The apparatus of claim 1 further comprising: the target tracking system providing information enabling the creation of a laser firing control signal.
 3. The apparatus of claim 1 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source directed to intersect a point on a projected delivery path of the target, having a respective oppositely disposed light detector detecting the passage of the target through the respective point.
 4. The apparatus of claim 2 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source directed to intersect a point on a projected delivery path of the target, having a respective oppositely disposed light detector detecting the passage of the target through the respective point.
 5. The apparatus of claim 1 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source and a detector comprising a linear array of a plurality of photo-sensitive elements aligned to a coordinate axis, the light from the light source intersecting a projected delivery path of the target.
 6. The apparatus of claim 2 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source and a detector comprising a linear array of a plurality of photo-sensitive elements aligned to a coordinate axis, the light from the light source intersecting a projected delivery path of the target.
 7. The apparatus of claim 3 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 8. The apparatus of claim 4 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 9. The apparatus of claim 5 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 10. The apparatus of claim 6 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 11. The apparatus of claim 3 further comprising: the droplet detectors comprise a plurality of droplet detectors each operating at a different light frequency.
 12. The apparatus of claim 4 further comprising: the droplet detector comprises a plurality of droplet detectors each operating at a different light frequency.
 13. The apparatus of claim 1, further comprising: the target tracking system comprising: a droplet detector comprising a camera having a field of view and a two dimensional array of pixels imaging the field of view.
 14. The apparatus of claim 2, further comprising: the target tracking system comprising: a droplet detector comprising a camera having a field of view and a two dimensional array of pixels imaging the field of view.
 15. A laser produced plasma EUV source comprising: an electrostatic plasma containment apparatus providing an electric plasma confinement field at or near a target ignition site at the time of ignition.
 16. The apparatus of claim 15 further comprising: a target tracking system providing a signal enabling control of the electrostatic plasma containment apparatus.
 17. An EUV light source comprising: a vessel; an EUV producing plasma generator; a collector focusing produced EUV light to an intermediate focus at one end of the vessel; an intermediate wall within the vessel between the plasma generator and the intermediate focus, the intermediate wall having an EUV light passage therein and separating the vessel into a zone of a first pressure and a zone of a second pressure; the EUV opening having therein a low pressure trap comprising passages for focused EUV light and constructed to maintain the pressure drop across the low pressure trap due to the difference between the first pressure and the second pressure.
 18. The apparatus of claim 17 further comprising: the low pressure trap comprises a section of a solid sphere having focused fine light passages formed therein.
 19. An EUV light source having a discrete target formation system providing targets at regular intervals, comprising: a first target tracking system providing outputs indicative of the tracking of a target from the target formation system, the target tracking system outputs comprising a target position and trajectory; a target steering system; a feedback and control system utilizing target position and trajectory outputs to provide inputs to the target steering system to enable the target steering system to steer the target to a desired target ignition site.
 20. The apparatus of claim 19 further comprising: a second target tracking system providing outputs indicative of the tracking of a target from the target steering system; the feedback and control system utilizing the outputs of the second target tracking system to generate a laser firing control signal.
 21. The apparatus of claim 19 further comprising: the target steering system comprises a target aiming mechanism and a target acceleration mechanism.
 22. The apparatus of claim 20 further comprising: the target steering system comprises a target aiming mechanism and a target acceleration mechanism.
 23. The apparatus of claim 19 further comprising: the first and second target tracking systems comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering system and the target steering system and the desired target ignition site.
 24. The apparatus of claim 20 further comprising: the first and second target tracking systems comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering system and the target steering system and the desired target ignition site.
 25. The apparatus of claim 21 further comprising: the first and second target tracking systems comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering system and the target steering system and the desired target ignition site.
 26. An EUV light source comprising a moving target plasma source and a pulsed laser plasma beam formation mechanism, wherein the respective target and the pulsed laser beam intersect at a desired target ignition site with an accuracy of about ±10 μm, to create a plasma comprising: a magnetic plasma confinement mechanism creating a magnetic field in the vicinity of the target ignition site to confine the plasma to the target ignition site.
 27. An EUV light source comprising a moving target plasma source and a pulsed laser plasma beam formation mechanism, wherein the respective target and the pulsed laser beam must intersect at a desired target ignition site with an accuracy of about ±10 μm, to create a plasma, comprising: a pulsed magnetic plasma containment mechanism creating a magnetic field in the vicinity of the target ignition site substantially coinciding with the existence of the plasma to contain the plasma to the target ignition site during the existence of the plasma.
 28. The apparatus of claim 26 further comprising: a target tracking system providing information enabling the control of the magnetic plasma confinement mechanism.
 29. The apparatus of claim 27 further comprising: a target tracking system providing information enabling the control of the magnetic plasma confinement mechanism.
 30. An EUV light source comprising: a pulsed laser means for providing laser pulses at a selected pulse repetition rate focused at a desired target ignition site; a target formation means for forming discrete targets at a selected interval coordinated with the laser pulse repetition rate; a target steering means intermediate the target formation means and the desired target ignition site; a target tracking means for providing information about the movement of target between the target formation means and the target steering means, and for enabling the target steering means to direct the target to the desired target ignition site.
 31. The apparatus of claim 30 further comprising: the target tracking means including means for providing information enabling the creation of a laser firing control signal.
 32. The apparatus of claim 30 further comprising: the target tracking means comprising: a droplet detector comprising a collimated light source directed to intersect a point on a projected delivery path of the target, having a respective oppositely disposed light detector detecting the passage of the target through the respective point.
 33. The apparatus of claim 31 further comprising: the target tracking means comprising: a droplet detector comprising a collimated light source directed to intersect a point on a projected delivery path of the target, having a respective oppositely disposed light detector detecting the passage of the target through the respective point.
 34. The apparatus of claim 30 further comprising: the target tracking means comprising: a droplet detector comprising a collimated light source and a detector comprising a linear array of a plurality of photo-sensitive elements aligned to a coordinate axis, the light from the light source intersecting a projected delivery path of the target.
 35. The apparatus of claim 31 further comprising: the target tracking means comprising: a droplet detector comprising a collimated light source and a detector comprising a linear array of a plurality of photo-sensitive elements aligned to a coordinate axis, the light from the light source intersecting a projected delivery path of the target.
 36. The apparatus of claim 32 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 37. The apparatus of claim 33 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 38. The apparatus of claim 34 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 39. The apparatus of claim 35 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 40. The apparatus of claim 32 further comprising: the droplet detectors comprise a plurality of droplet detectors each operating at a different light frequency.
 41. The apparatus of claim 33 further comprising: the droplet detector comprises a plurality of droplet detectors each operating at a different light frequency.
 42. The apparatus of claim 30, further comprising: the target tracking means comprising: a droplet detector comprising a camera having a field of view and a two dimensional array of pixels imaging the field of view.
 43. The apparatus of claim 31, further comprising: the target tracking means comprising: a droplet detector comprising a camera having a field of view and a two dimensional array of pixels imaging the field of view.
 44. A laser produced plasma EUV source comprising: an electrostatic plasma containment apparatus providing an electric plasma confinement field at or near a target ignition site at the time of ignition.
 45. The apparatus of claim 44 further comprising: a target tracking means including a means for providing a signal enabling control of the electrostatic plasma containment apparatus.
 46. An EUV light source comprising: a vessel; an EUV producing plasma generating means; a collector focusing produced EUV light to an intermediate focus at one end of the vessel; an intermediate wall within the vessel between the plasma generator and the intermediate focus, the intermediate wall having an EUV light passage therein and separating the vessel into a zone of a first pressure and a zone of a second pressure; the EUV opening having therein a low pressure trap means comprising passages for focused EUV light and means for maintaining the pressure drop across the low pressure trap due to the difference between the first pressure and the second pressure.
 47. The apparatus of claim 46 further comprising: the low pressure trap means comprises a section of a solid sphere having focused fine light passages formed therein.
 48. An EUV light source having a discrete target formation means for forming targets at regular intervals, comprising: a first target tracking means for providing outputs indicative of the tracking of a target from the target formation means, the target tracking means outputs comprising a target position and trajectory; a target steering means; a feedback and control means for utilizing target position and trajectory outputs to provide inputs to the target steering means to enable the target steering means to steer the target to a desired target ignition site.
 49. The apparatus of claim 48 further comprising: a second target tracking means providing outputs indicative of the tracking of a target from the target steering system; the feedback and control means utilizing the outputs of the second target tracking system for generating a laser firing control signal.
 50. The apparatus of claim 48 further comprising: the target steering means comprises a target aiming means and a target acceleration means.
 51. The apparatus of claim 49 further comprising: the target steering means comprises a target aiming means and a target acceleration means.
 52. The apparatus of claim 48 further comprising: the first and second target tracking means comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering mechanism and the target steering mechanism and the desired target ignition site.
 53. The apparatus of claim 49 further comprising: the first and second target tracking means comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering mechanism and the target steering mechanism and the desired target ignition site.
 54. The apparatus of claim 50 further comprising: the first and second target tracking means comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering mechanism and the target steering mechanism and the desired target ignition site.
 55. An EUV light source comprising a moving target plasma source and a pulsed laser plasma beam formation mechanism, wherein the respective target and the pulsed laser beam intersect at a desired target ignition site with an accuracy of about ±10 μm, to create a plasma comprising: a magnetic plasma confinement means for creating a magnetic field in the vicinity of the target ignition site to confine the plasma to the target ignition site.
 56. An EUV light source comprising a moving target plasma source and a pulsed laser plasma beam formation mechanism, wherein the respective target and the pulsed laser beam must intersect at a desired target ignition site with an accuracy of about ±110 μm, to create a plasma, comprising: a pulsed magnetic plasma containment means for creating a magnetic field in the vicinity of the target ignition site substantially coinciding with the existence of the plasma to contain the plasma to the target ignition site during the existence of the plasma.
 57. The apparatus of claim 55 further comprising: a target tracking system providing information enabling the control of the magnetic plasma confinement mechanism.
 58. The apparatus of claim 56 further comprising: a target tracking system providing information enabling the control of the magnetic plasma confinement mechanism.
 59. An EUV light producing method comprising: utilizing a pulsed laser, providing laser pulses at a selected pulse repetition rate focused at a desired target ignition site; forming discrete targets at a selected interval coordinated with the laser pulse repetition rate; utilizing a target steering system intermediate the formation of the target and the desired target ignition site; utilizing a target tracking system providing information about the movement of target between the target formation and the target steering system, and for enabling the target steering system to direct the target to the desired target ignition site.
 60. The method of claim 59 further comprising: utilizing the target tracking system, providing information enabling the creation of a laser firing control signal.
 61. The method of claim 59 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source directed to intersect a point on a projected delivery path of the target, having a respective oppositely disposed light detector detecting the passage of the target through the respective point.
 62. The method of claim 60 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source directed to intersect a point on a projected delivery path of the target, having a respective oppositely disposed light detector detecting the passage of the target through the respective point.
 63. The method of claim 59 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source and a detector comprising a linear array of a plurality of photo-sensitive elements aligned to a coordinate axis, the light from the light source intersecting a projected delivery path of the target.
 64. The method of claim 60 further comprising: the target tracking system comprising: a droplet detector comprising a collimated light source and a detector comprising a linear array of a plurality of photo-sensitive elements aligned to a coordinate axis, the light from the light source intersecting a projected delivery path of the target.
 65. The method of claim 61 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 66. The method of claim 62 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 67. The method of claim 63 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 68. The method of claim 64 further comprising: at least one of the droplet detectors comprises a plane-intercept detection device.
 69. The method of claim 61 further comprising: the droplet detectors comprise a plurality of droplet detectors each operating at a different light frequency.
 70. The method of claim 62 further comprising: the droplet detector comprises a plurality of droplet detectors each operating at a different light frequency.
 71. The method of claim 59, further comprising: the target tracking system comprising: a droplet detector comprising a camera having a field of view and a two dimensional array of pixels imaging the field of view.
 72. The method of claim 60, further comprising: the target tracking system comprising: a droplet detector comprising a camera having a field of view and a two dimensional array of pixels imaging the field of view.
 73. A laser produced plasma EUV light producing method comprising: utilizing an electrostatic plasma containment apparatus, providing an electric plasma confinement field at or near a target ignition site at the time of ignition.
 74. The method of claim 73 further comprising: utilizing a target tracking system, providing a signal enabling control of the electrostatic plasma containment apparatus.
 75. An EUV light producing method comprising: utilizing a plasma producing vessel having an intermediate wall within the vessel between having an EUV light passage therein and separating the vessel into a zone of a first pressure and a zone of a second pressure; providing in the wall a low pressure trap comprising passages for focused EUV light and maintaining the pressure drop across the low pressure trap due to the difference between the first pressure and the second pressure.
 76. The method of claim 75 further comprising: the low pressure trap comprises a section of a solid sphere having focused fine light passages formed therein.
 77. An EUV light producing means utilizing a discrete target formation system forming targets at regular intervals, comprising: utilizing a first target tracking system, providing outputs indicative of the tracking of a target from the target formation system, the target tracking system outputs comprising a target position and trajectory; utilizing a target steering system; utilizing a feedback and control system, utilizing the target position and trajectory outputs to provide inputs to the target steering system to enable the target steering system to steer the target to a desired target ignition site.
 78. The method of claim 77 further comprising: utilizing a second target tracking system, providing outputs indicative of the tracking of a target from the target steering system; utilizing the feedback and control system, utilizing the outputs of the second target tracking system for generating a laser firing control signal.
 79. The method of claim 77 further comprising: the target steering system comprises a target aiming mechanism and a target acceleration mechanism.
 80. The method of claim 78 further comprising: the target steering system comprises a target aiming means and a target acceleration means.
 81. The method of claim 77 further comprising: the first and second target tracking systems comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering mechanism and the target steering mechanism and the desired target ignition site.
 82. The method of claim 78 further comprising: the first and second target tracking systems comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering mechanism and the target steering mechanism and the desired target ignition site.
 83. The apparatus of claim 79 further comprising: the first and second target tracking systems comprising an x and a y axis position detector and a z plane passage detector respectively intermediate the target delivery system and the target steering mechanism and the target steering mechanism and the desired target ignition site.
 84. An EUV light producing method comprising using a moving target plasma source and a pulsed laser plasma beam formation mechanism, wherein the respective target and the pulsed laser beam intersect at a desired target ignition site with an accuracy of about ±10 μm, to create a plasma comprising: utilizing a magnetic plasma confinement mechanism, creating a magnetic field in the vicinity of the target ignition site to confine the plasma to the target ignition site.
 85. An EUV light producing method comprising using a moving target plasma source and a pulsed laser plasma beam formation mechanism, wherein the respective target and the pulsed laser beam must intersect at a desired target ignition site with an accuracy of about ±10 μm, to create a plasma, comprising: utilizing a pulsed magnetic plasma containment mechanism, creating a magnetic field in the vicinity of the target ignition site substantially coinciding with the existence of the plasma to contain the plasma to the target ignition site during the existence of the plasma.
 86. The method of claim 84 further comprising: utilizing a target tracking system, providing information enabling the control of the magnetic plasma confinement mechanism.
 87. The apparatus of claim 85 further comprising: utilizing a target tracking system providing information enabling the control of the magnetic plasma confinement mechanism.
 88. An LPP EUV light source comprising: a collector mirror comprising a multi-layer reflecting surface; at least one component in closed enough proximity to the plasma produced in the LPP EUV light source to be eroded by the effects of the plasma; a coating on the at least one component that is not damaging to the multi-layer reflecting surface if sputtered onto the multi-layer reflective surface.
 89. The apparatus of claim 88 further comprising: the multi-layer reflecting surface is coated with the same coating as the at least one component.
 90. The apparatus of claim 88 further comprising: the multi-layer reflecting surface includes layers of the same material as the coating of the at least one component.
 91. An LPP EUV light source comprising: an LPP EUV chamber; a driving laser producing a driving laser beam directed at a target to produce within the chamber the plasma for the LPP EUV light source; an input window through which the driving laser beam enters the chamber; a heater element heating the input window.
 92. An LPP EUV light source comprising: a target formation system comprising a nozzle from which a target droplet or a liquid stream that eventually forms a target droplet is ejected along a target formation axis; a target tracking system detecting the position of the target droplet at one or more positions in the target flight path intermediate the nozzle and the vicinity of a a desired target ignition site coordinated with the arrival of an irradiating beam at the desired target ignition site and detecting an error in that flight path and/or an error in the position of the target droplet vis-á-vis the desired target ignition site at the arrival time of the irradiating beam; a target formation system tilting mechanism tilting the target formation axis based upon the detected error to decrease the error in a subsequent target droplet position vis-á-vis the desired target ignition site at the arrival time of the irradiating beam. 